A honeycomb structure made of ceramics has been widely used for catalyst carriers, various filters, etc. Recently, the structure has particularly attracted the attention as a diesel particulate filter (DPF) for trapping particulate matters (PM) discharged from a diesel engine.
Such a honeycomb structure can generally be obtained by kneading a raw material composition obtained through addition of an auxiliary forming agent and various addition agents to dispersion media, such as a ceramic material and water to form a clay, then extruding the clay into a honeycomb-shaped formed body (honeycomb formed body), drying this honeycomb formed body, and then firing the honeycomb formed body.
As means for drying a honeycomb formed body, there have been known a natural drying method in which the honeycomb formed body is simply left under a room temperature condition, a hot-air drying method in which the honeycomb formed body is dried by hot air generated with a gas burner, and a dielectric drying method in which the honeycomb formed body is dried utilizing high-frequency energy generated by causing a current to flow between electrodes provided at an upper side and a lower side of the honeycomb formed body, but recently, a microwave drying method utilizing microwaves has been performed in place of these drying methods, or in combination with them.
The microwave drying is performed, for example, after an electric field distribution in a drying furnace is made uniform prior to then placing the honeycomb formed bodies to be dried therein. As means for uniformizing the electric field distribution, there is known a method for adjusting a shape and placement of an antenna that radiates microwaves, and a method for using a stirrer fan, etc. As prior art documents on the microwave drying method, Japanese Patent Laid-Open No. 2002-283330, Japanese Patent Laid-Open No. 2004-167809, International Publication Pamphlet 2005/023503, Japanese Patent Laid-Open No. 2000-44326, and Japanese Utility Model Laid-Open No. 1986-13497 are cited.